1. Field of the Invention
This invention relates to a photochromic material which is of interest as one of photon mode recording materials and a method for preparing the same. It also relates to a photochromic thin film. It further relates to a clay thin film capable of adsorbing functional molecules such as photochromic molecules as a guest, a clay thin film having functional molecules adsorbed thereto, and a method for preparing the same.
2. Prior Art
Great attention is now paid to photon mode recording materials as a next generation of optical recording material capable of super-high density recording. Active research efforts have been made on photochromic materials although the technology has not matured to a practically acceptable level because of many outstanding problems including colorant stability, non-destructive reading, and repetitive durability.
Spirobenzopyrans which form a class of photochromic materials have characteristics including a great difference in absorption spectrum before and after light exposure and development of various colors by a choice of substituents, but suffer from low thermal stability. Then attempts have been made to disperse spiro-benzopyrans in resin binders, but at the sacrifice of the rate of recording, reading and erasing and hence, the performance as optical recording material.
For this and other reasons, Japanese Patent Publication (JP-B) No. 16776/1992 discloses an optical recording material having spiro-benzopyrans dispersed in a film having a composite layered structure including a clay and a bimolecular membrane resulting from an ionic complex of a cationic surfactant having a linear higher alkyl group and an anionic layer-structured clay compound. The anionic layer-structured clay compound used in this publication is an ion exchangeable silicate capable of intercalating a cationic organic compound between layers thereof, for example, montmorillonite. Several spiro-benzopyrans are described, but anionic ones are not disclosed. It is described that the bimolecular membrane exhibits phase transition behavior between clay layers and that the film assumes a lamellar crystal state having low molecular kinetic energy at low temperatures below the phase transition temperature, but a smectic liquid crystal state having high molecular kinetic energy at high temperatures above the phase transition temperature. This enables rapid recording at high temperatures above the phase transition temperature and stable storage of recorded information at low temperatures below the phase transition temperature. However, only a limited degree of freedom of design is allowed because the combination of a cationic surfactant and an anionic layer-structured clay compound capable of forming an ionic complex is restricted.
According to Japanese Patent Application Kokai (JP-A) No. 264246/1990, spiro-pyrans are readily carried on a solid phase matrix in a stable manner by using cationic spiro-pyrans and silicate minerals having a three layer structure (smectite group mineral) such as montmorillonite as the matrix. This proposal, however, fails to improve the stability of anionic and neutral spiro-pyran compounds themselves, limits being imposed on the selection of substituents and color generation.
JP-A 151142/1992 discloses a method of forming a thin film of an ionic complex by forming an ionic complex from a photochromic substance having an anionic group and a cationic surfactant and forming a thin film of the ionic complex by the Langmuir-Brodgett technique. With the Langmuir-Brodgett technique, it is difficult to form a uniform film of a large surface area and to produce it on an industrial large scale.
There is a need to improve the thermal stability of anionic and neutral photochromic molecules for improving their characteristics as optical recording material.
The invention further relates to a clay thin film capable of adsorbing functional molecules such as photochromic molecules as a guest, a clay thin film having functional molecules adsorbed thereto as the guest, and a method for preparing the same.
Many attempts have been made to insert organic functional substances between layers of inorganic layer-structured crystal materials such as clay minerals so as to form intercalation compounds exhibiting new functions. The materials are in powder form in most of these attempts. For commercially acceptable application of such intercalation compounds, it is important to form a thin film which can find a wider range of application. If a transparent thin film is available, a wide range of optical application is expectable.
In JP-B 16776/1992 referred to above, the intercalation compound is cast to form a film of 0.01 mm thick. In JP-A 264246/1990 referred to above, the matrix having the spiro-pyran adsorbed thereto is subject to centrifugal separation. The precipitate is passed through a membrane filter under vacuum and dried under vacuum at room temperature. A pale yellow clay/spiro-pyran composite film is then stripped from the filter. However, such a thin film of the smectite mineral is less transparent. Where it is desired to use the thin film in an optical application, for example, as a photochromic thin film, it offers no satisfactory S/N. Since it is not expectable from the prior art techniques to improve the stability of anionic and neutral spiro-pyran compounds themselves, limits are imposed on the selection of substituents and color generation.